1. Field of the Invention
The present invention relates to an FTTH (fiber to the home) system, and more particularly to a system for transmitting telephone voice data to a subscriber terminal using an optical line.
2. Description of the Related Art
The traditional telephone system uses a copper wire as a transmission medium for transmission of an analog voice data between a TDX switching system and a subscriber's telephone. However, as the subscribers' demand for digital data transmission increases, a digital data modulation method using the existing copper wire for the telephone has been developed. The digital data modulation method is classified into one method of transmitting an analog band and a digital transmission band separately, and another method in which a telephone voice signal is converted into a digital signal to enable the digital transmission. The former method refers to a xDSL technology, such as ADSL, VDSL, etc., that processes uplink/downlink data using a frequency modulation technique, except for a frequency of analog voice data. The xDSL technology separates the baseband telephone voice data from digital data through a splitter at a subscriber end. Meanwhile, the later method involves converting the analog telephone voice data into digital data and, then transmitting the digital data from the telephone office to PBX of MDF or a small PBX (i.e., keyphone) of a subscriber's house.
FIG. 1 is a view illustrating an example of the conventional apparatus for converting the telephone voice signal into the digital signal and performing the digital transmission on the whole network.
Referring to FIG. 1, a DAA (Data Access Arrangement) 100, which receives an analog voice signal 106 that includes analog signaling data, transfers the analog signaling data to a control unit 101, and converts the analog voice signal 106 except for the analog signaling data into digital voice data, and then transfers digital data 105 that includes the signaling signal and the digital voice data to a downlink 103 of a PCM bus for a subsequent transmission to a subscriber end. The DAA receives digital data transferred from the subscriber end through an uplink 104 of the PCM bus, and converts the received digital data into an analog voice signal.
Meanwhile, an SLIC (subscriber line interface unit) 107 of the subscriber end performs similar function as the DAA 100. That is, the SLIC 107 receives and converts the digital data 105 transferred to the downlink 103 of the PCM bus into analog voice data 108, and receives and converts the analog voice data from the subscriber into digital voice data in order to transfer the converted digital voice data to the uplink 104 of the PCM bus.
FIG. 2 is a view exemplifying signals input/output between a TDX switching system 200 of a phone company and a home telephone in the existing telephone communications using analog voice data.
The signals transmitted/received between the TDX switching system 200 and the home telephone may be an on-off hook signal 201 produced when a user picks up or hangs up a handset. A dial tone signal 202 is produced when the user picks up the handset. An address signal 203 is produced when the user dials the other party's phone number. A ring-back tone signal 204 signifies that the user can hear while the opposite party's telephone is ringing according to a user's call connection attempt, and a busy tone signal 205 us produced while the other party's telephone line is busy. A DTx signal 206 or DRx signal 207 corresponding to voice data is produced while speaking, and a ring signal 204 is produced while the other party is making a call to the user, etc.
Among the signaling information described above, the signaling that does not correspond to the voice bandwidth (of about 300˜3000 Hz) is called out-of-band signaling information, and it is impossible to transmit this information through an analog-to-digital (A/D) conversion. If the transmission is performed according to the conventional method as described above, a complicated process must be performed in a manner that the telephone office transmits a digital type signal that is multiplexed in the form of T1 or E1, and the system in MDF or at home must convert the transmitted digital signal into an analog signal. This method is not suitable to the optical communication system whereby several voice data are transmitted through one optical fiber in a point-to-point manner from the center base station to the subscriber. As a result, there is a need for an improved optical communication system that can support a telephone service.